A Bi-Directional Freshwater Plume in the California Current System

Barbara M. Hickey

School of Oceanography, University of Washington, Seattle, 98195-07940, USA
EMAIL: bhickey@u.washington.edu

Freshwater plumes have important effects on marine ecosystems: in the presence of a plume, stratification, nutrient pathways, light and circulation patterns are significantly altered from patterns that occur under the influence of wind and ambient currents alone. The historical picture of the plume from the Columbia River consists of a freshwater plume oriented southwest offshore of the Oregon shelf in summer, north or northwest along the Washington shelf in winter. The plume is shallow, with strong frontal regions and current jets near its edges and eddy-like, retentive regions within its body. Both recent and historical data support a picture quite different from the historical seasonal pattern: specifically, the plume is frequently present up to 150 km north of the river mouth on the Washington shelf from spring to fall, even during periods of upwelling. During some years, fresher water and stronger fronts occur in spring and summer on the Washington shelf than during winter. Examples in which the Columbia freshwater plume impedes upwelling of nutrients by capping the upwelling water over the inner shelf are shown. Although the Columbia plume exports little nitrate from the estuary to the coast, it is a major source of silicate and also iron. Thus the position of the plume, as well as where it deposits its iron-rich sediment load is likely to play a first order role in productivity on nearby shelves. Evidence suggests that the Columbia River plume may also provide a barrier to the transport of harmful algal blooms to coastal beaches in summer and early fall.